Continuous liquid jets are commonly used for cutting and disintegration of various materials, for cleaning and removal of surface layers and coatings. Generating of sufficiently high pressure pulsations in pressure liquid upstream from the nozzle exit (so called modulation) enables to generate a pulsating liquid jet that emerges from the nozzle as a continuous liquid jet and it not forms into pulses until certain standoff distance from the nozzle exit. The advantage of such a pulsating jet compared to the continuous one consists in fact that the initial impact of pulses of pulsating jet on the target surface generates impact pressure that is several times higher than stagnation pressure generated by the impact of continuous jet under the same conditions. In addition, the impact of pulsating jet induces also fatigue stress in target material due to cyclic loading of the target surface. This further improves an efficiency of the pulsating liquid jet compared to the continuous one.
At present, several types of devices intended for generation of pulsating liquid jets are available. Internal mechanical flow modulators are mechanical devices integrated in the nozzle. They are formed essentially by channeled rotor placed upstream the nozzle exit. The rotor cyclically changes resistance of flow by its rotation and thus modulates velocity of the jet emerging from the nozzle (E. B. Nebeker: Percussive Jets—State-of-the-Art, Proceedings of the 4th U.S. Water Jet Symposium, WJTA, St. Louis, 1987). The main shortcoming of the above mentioned principle is very low lifetime of moving components in the nozzle.
Modulation of continuous liquid jets by Helmholtz oscillator is based on the fact that changes in flow cross-section and/or flow discontinuities provoke periodical pressure fluctuations in flowing liquid (Z. Shen & Z. M. Wang: Theoretical analysis of a jet-driven Helmholtz resonator and effect of its configuration on the water jet cutting property, Proceedings of the 9th International Symposium on Jet Cutting Technology, BHRA, Cranfield, 1988). The same physical principle is used in so-called self-resonating nozzles. Certain type of shock pressure is developed when liquid flows over exit of resonating tube. The shock pressure is carried back to the tube inlet where it creates standing wave by addition with pressure pulsations. If frequency of the shock pressure corresponds to natural frequency of the flow, pressure resonance occurs and the jet starts to create discrete annular vortexes that result in generation of cavitations and/or pulses. (G. L. Chahine et al.: Cleaning and cutting with self -resonating pulsed water jets, Proceedings of the 2nd U.S. Water Jet Symposium, WJTA, St. Louis, 1983). The primary disadvantage of the above mentioned devices is low depth of modulation of liquid jet.
An ultrasonic nozzle for modulation of high-speed water jet is based on a vibrating transformer placed upstream in the vicinity of the nozzle exit in such a way that pressurized fluid flows through annulus between the transformer and nozzle wall. The vibrating transformer is connected to magnetostrictive and/or piezoelectric transducer. The transformer generates highly intensive ultrasound field upstream of the nozzle exit that modulates high-speed water jet escaping from the nozzle (M. M. Vijay: Ultrasonically generated cavitating or interrupted jet, U.S. Pat. No. 5,154,347, 1992). High wear of the tip of vibrating transformer due to intense cavitational erosion, increased dimensions and weight of cutting tool rank among the most important drawbacks of the above mentioned device. The level of modulation is strongly dependent on the position of the tip of the vibrating transformer with respect to the nozzle exit. In addition to that, the ultrasonic nozzle device does not allow utilizing of existing cutting tools for continuous water jets, which significantly increases costs of its implementation in industrial practice.